As it is well known, diving cylinders are containers of essentially cylindrical shape with a capacity usually lower than 18 liters, which are structured to contain breathable air with nominal maximum pressure traditionally in the range between 200 to 300 bar.
This type of cylinders is usually filled by means of special high pressure compressors which are structured in such a way as to be able to aspirate air at ambient pressure, and deliver at the output an appropriately filtered and dehumidified air flow, with nominal pressure greater than 200-300 bar.
On some circumstances, the diving cylinders can also be filled with breathable gas mixtures where the stoichiometric ratios between oxygen, nitrogen and any other gases are radically different from those one typical of the breathable air.
In these cases, the filling of the cylinder is carried out with radically changing systems depending on the composition of the breathable gas mixture that must be fed into the cylinder.
More specifically, in case of breathable gas mixtures formed by oxygen and nitrogen with oxygen percentage greater than that one present in the air (that is, with an oxygen percentage greater than 21%, commonly known in the market also with the term Nitrox), the cylinders are filled through high pressure compressors which present, at the intake of the sucking manifold, one or more nozzles which are able of injecting pure oxygen into the air flow at ambient pressure that flows along the sucking manifold towards the aspiration mouth of the pumping group.
These nozzles are connected with pure oxygen tank at high pressure by interposing a pressure reducer and a solenoid valve which is driven by a dedicated electronic control unit in such a way as to pump in into the sucking manifold of the compressor a flow of pure oxygen substantially constant over time, whose value is a function of the percentage of oxygen that the breathable gas mixture must have at the output of the compressor.
In other words the electronic control unit calculates, as a function of the oxygen and nitrogen percentages attributed to the breathable gaseous mixture, what is the correct pure oxygen flow to be sent to the sucking manifold of the compressor in order to get, at the output of the high pressure compressor, the breathable gas mixture with the required oxygen and nitrogen percentages, and then drives the solenoid valve placed upstream the nozzles so that the pure oxygen flow directed towards the nozzles is always equal to the calculated oxygen flow.
In case, instead, of breathable gaseous mixtures with low oxygen percentages and high helium percentages (i.e. breathable gaseous mixtures formed by oxygen, nitrogen and helium in percentages other than those ones found in the air, commonly known in the market also with the term Trimix), used by some divers for the most demanding diving, the cylinders are usually filled up through subsequent dumps by directly feeding into the cylinder, separately and in successive stages, the required amounts of oxygen, nitrogen and helium.
Basically, the breathable gas mixture with the required composition is made directly into the cylinder by manually entering the latter, separately and in successive stages, the appropriate amounts of oxygen, nitrogen and helium.
It is briefly reminded herein that the amount of oxygen within the Trimix gas mixture must not have a partial pressure higher than 160 kPa. Pure oxygen can be used at a maximum depth of 6 meters: beyond such a depth oxygen is toxic to the central nervous system of the human being.
In addition, nitrogen is a heavy gas and beyond a depth of 30 meters (400 kPa) becomes narcotic because it enters solution in the lipid layers of the body that make up the neurons and interferes with normal signals, giving a sense of drunkenness to the human being: the more pressure increases this effect becomes stronger.
For its part, however, helium is, as it is well-known, a lighter and more inert gas and enters solution in the tissues present in the human body more quickly but at the same time exits from tissues at the same speed: the presence of helium in the gaseous mixture decreases the amount of gases contained in the air (oxygen and nitrogen), thus reducing the narcotic effect of nitrogen and giving more lucidity during deep dives.
Unfortunately, the filling system through subsequent dumps currently requires the availability of large tanks of oxygen, nitrogen and helium with pressures greater than 200-300 bar, i.e. with pressures higher than the maximum nominal filling capacity of the cylinder, with the costs that this involves.
Moreover, addition of pure oxygen at high pressure inside the cylinder is a particularly dangerous operation that can cause explosions, with all the operative problems that this implies.
Despite the above drawbacks, the filling system for subsequent dumps remains at present substantially the most widespread procedure for obtaining, quickly and with acceptable error margins, breathable gas mixtures with the desired stoichiometric composition.
There is, indeed, an alternative system, of manual type, for filling the cylinder: it employs a mixer and electronic control performed only on the percentage of gas (nitrogen or helium) and on the driving of the solenoid valve solely dedicated to the gas opening and closure, while for regulating the flow manual regulators or chokes, thus manoeuvred by the user, are provided.
Up to now, indeed, the attempts to accurately get a prefixed breathable gaseous mixture by injecting into the air flow entering the intake manifold of the compressor, in parallel, a constant flow of pure oxygen and a constant flow of pure helium have not given the results hoped for: the breathable gas mixtures obtained with such a system are, indeed, affected by random errors in oxygen, nitrogen and helium fractions often exceeding 10%, which represents an absolutely irreconcilable margin of error with an industrial use.
These disadvantages have been overcome by the invention described in the Italian patent no. 1415144 filed and granted in the name of the same applicant of the present invention, which describes an apparatus for feeding breathable gas mixtures with pre-established stoichiometric ratios of oxygen, nitrogen and helium at the intake of a high pressure compressor for filling diving cylinders, which allows to get, at the output of high pressure compressor itself breathable gas mixtures with errors in oxygen, nitrogen and helium fractions which are minimal, acceptable for an industrial, technical and professional use, however significantly lower than the state of the art of that time.
The technical teachings, the purpose and the whole technical matter contained in the aforesaid Italian patent no. 1415144 are fully incorporated in the present invention as base reference and constitute the starting point (so-called the closest prior art) thereof.
However, even the apparatus for feeding gas mixtures at the intake of a high pressure compressor described in the Italian patent no. 1415144 and also proposed on the market by the applicant presents some operative limitations, mainly related to the inherent constructional features of the two servo-controlled throttling valves (or flow adjusters) driven by the electronic control unit and interposed respectively between the first gas intake device and the source (typically a 200 bar tank) of pressurized helium and between the second gas intake device and the source (again typically a 200 bar reservoir) of pressurized oxygen.
Indeed, the structural and constructive limitation of the servo-controlled throttling valves, determined by the opening through which the gas—oxygen or helium—flow passes through them, prevents to get determined flow rates (in liters/minute) which are discharged to the compressor and which the latter, in certain operative or use conditions, inevitably and necessarily requires.
In essence, therefore, each of the servo-controlled throttling valves of which the apparatus of the known type described in the Italian patent no. 1415144 is provided (and which, it is stated precisely, are designed to operate at pressures not exceeding 8 bar) allows to achieve a certain maximum flow rate (in liters/minute) beyond which, constructively, it does not anyway allow to go, although the use, represented by the compressor at high pressure, sometimes requires a greater flow than that one obtainable with these pressure values.
In addition, the pressure reducer associated with the gas cylinder (both for nitrogen and for helium) which allows to adjust the pressure of the gas to be mixed, ranging from 200 or 300 bar to values between 1 and 8 bar, presents the disadvantage of providing gas at a fixed pressure, still manually adjusted by the operator or the user.
To meet the aforesaid incremental operative requirements of the high pressure compressor, the user is then obliged to intervene manually on the pressure reducer just of the gas source—oxygen tank or helium tank—installed as known in its upper part, opening it of what is necessary to increase the gas pressure and allow to supply to the compressor a required gas flow (for Bernoulli's equation, the flow is, indeed, directly proportional to the pressure), which is in any case fixed.
This inevitably negatively affects timing and precision of the filling operations of the diving cylinders, resulting in the user's dissatisfaction or incomplete satisfaction.